This project studies the nature of family and household relationships among persons of Mexican origin in the United States between 1880 and 1990, with specific tests of competing theories about assimilation trajectories. Theoretical models about the assimilation of immigrants and their descendants have explicit, but rarely examined, historical assumptions. Although the Mexican family has been central to the scholarly literature, a lack of reliable historical data on the Mexican origin population has made it difficult to assess assimilation processes in this important group. The research will redress these gaps by: a) developing better theory on assimilation by grounding it in historical data; b) providing a new understanding of the assimilation trajectories of Mexican origin families; and c) connecting these findings to likely outcomes in the health status and life circumstances of the elderly, women, and children. The project s aims are: a) to construct the first reliable historical series on Mexican origin family/household structure between 1880 and 1990, using the Integrated Public Use Samples of the U.S. Censuses; b) to contrast this series to similar ones for other immigrants and their descendants, and for natives; c) to use the comparative data to test theories of familism, assimilation, and segmented assimilation. The project uses a data set that provides representative samples of Mexican origin persons between 1880 and 1990, comparative data sets to be constructed for non-Hispanic whites of immigrant and native origin, and similar data sets for Italians, Puerto Ricans, and African Americans. These series allow comparative analyses across time of household and family structure, controlling for generational status. A variety of regression analyses, including multinomial logistic regression on pooled data, will test the effects of ethnicity, generation, time, and period on family and household outcomes. Children, the elderly, and female-headed households will be the foci of the statistical analyses. The research is significant for scholarship and policy, given the importance of the Mexican origin population in U.S. society and the health and social implications of family structure and assimilation paths.